1. Field of the Invention
The invention relates to a process for reforming a naphtha stream using a surface treated zeolite catalyst.
2. Description of Related Art
Naphtha streams emerging from petrochemical refining processes generally comprise a mixture of C.sub.5 to C.sub.13 hydrocarbons which include about 15 to 40 wt. % of C.sub.6 to C.sub.11 aromatic compounds and the balance mostly a mixture of C.sub.5 to C.sub.11 aliphatic hydrocarbons, including mixed paraffins and mixed olefins.
It is well known in the art that such streams may be subjected to catalytic reforming to further enhance the more valuable aromatics content of the naphtha. In a typical reforming process, naphtha is passed over an acidic, medium pore zeolite catalyst, such as ZSM-5, which may also contain one or more dehydrogenation metals such as noble metals, under reforming conditions which include a temperature of 400-10000F., pressures of 50-300 psig, weight hourly space velocity of 0.5-25 and in the optional presence of hydrogen (H.sub.2 to oil mole ratio of about 0-10). In a typical reforming process, the reactions include dehydrogenation, dehydrocyclization, isomerization and hydrocracking. For example, the use of a zinc-modified ZSM-5 aluminosilicate as a reforming catalyst for light naphtha feedstock is disclosed by Fukase et al, "Catalysts in Petrochemical Refining and Petrochemical Industries 1995", 1996, pp 456-464.
The dehydrogenation reactions typically include dehydroisomerization of alkylcyclopentanes to aromatics, the dehydrogenation of paraffins to olefins, the dehydrogenation of cyclohexanes to aromatics and the dehydrocyclizaiton of acyclic paraffins and acyclic olefins to aromatics. The aromatization of the n-paraffins to aromatics is generally considered to be the most important because of the high octane rating of the resulting aromatic product. The isomerization reactions include isomerization of n-paraffins to isoparaffins, the hydroisomerization of olefins to isopraffins, and the isomerization of substituted aromatics. The hydrocracking reactions include the hydrocracking of paraffins and hydrodesulfurization of sulfur compounds in the feed stock.
Acidic zeolites of the HZSM-5 type are also well known catalysts for use in toluene disproportionation reactions wherein toluene or mixtures of toluene and methanol are fed over the catalyst under disproportionation/alkylation conditions. In many such processes, the catalyst is first treated with a silicon-containing compound or other material to reduce the surface acidity of the catalyst. This technique has been found to enhance selectivity of the disproportionation process towards the production of the more valuable para-xylene isomers, in contrast with the meta or ortho isomers. Examples of such processes are found in U.S. Pat. Nos. 4,950,835, 5,321,183 and 5,367,099.
U.S. Pat. No. 5,371,312 discloses a process for the conversion of hydrocarbons comprising passing a hydrocarbon stream over a zeolite which has been treated with an amino silane. When the conversion process is toluene disproportionation, the patent indicates that the catalyst may also contain a dehydrogenation metal such as platinum to reduce the amount of ethyl benzene by-product formed in the process.
In addition, U.S. Pat. No. 5,202,513 discloses the use of a galloalumino silicate catalyst of the ZSM-5 type containing gallium as part of the crystal structure which is treated with an alkali hydroxide, used as a reforming catalyst for naphtha-type feeds.
In an article by Y. S. Bhat et al., Appl. Catal. A, 130 (1995) L1-L4, it is disclosed that n-pentane aromatization over an MFI catalyst which has been silylated by vapor deposition of an organosilicone compound gives increased selectivity towards para-xylene production.
WO 96/03209 discloses a reforming process wherein a C.sub.5 -C.sub.9 paraffin or olefin feedstock is contacted under reforming conditions with a zeolite catalyst which has been modified with a platinum group component metal and a second metal selected from gallium, zinc, indium, iron, tin and boron. The publication indicates that the process leads to an increased yield of para-xylene and that the yield of para-xylene is further enhanced by pre-coking the catalyst prior to use in the reforming process.
One of the major drawbacks associated with the use of acidic medium pore zeolite catalysts in reforming process, as contrasted with disproportionation processes, is that an undesirable amount of molecular cracking takes place wherein a significant portion of molecules having 5 or more carbon atoms are degraded, rather than upgraded into more valuable products. As a result, quantities of low value C.sub.1 to C.sub.4 gases are produced, often in quantities of greater than about 25 wt % of the initial naphtha feedstream.
Accordingly, it is an object of this invention to provide a process for reforming a naphtha feed using a modified zeolite catalyst wherein the quantity of low value C.sub.1 to C.sub.4 gas by-product produced in the process is markedly reduced.
Another object of the invention is to provide a process for reforming a naphtha feed using a modified zeolite catalyst wherein the para-xylene content of the C.sub.8 aromatic product present in the reformate is produced in greater than an equilibrium-amount.